Generally, electronically switchable diplexers are key components for joining or separating signal paths of different frequency bands. They are for example used in signal generator applications because the usage of such diplexers allows for low-loss switching the signals generated by several signal generator modules of a signal generator, each module operating at another frequency band, to a common output of the signal generator according to a desired output frequency. Switchable diplexers can also be used in signal receivers, for instance in spectrum analyzers, where they allow switching of the received signals at the common input terminal into separate receiving paths according to their frequency.
EP 1 780 889 A1 discloses a switchable diplexer to be used for microwave and radio frequency applications. As the PIN-diodes of the switching elements of this switchable diplexer are directly coupled to the signal line, such a diplexer has a DC charge on the common terminal and the low frequency terminal DC charge could destroy circuit components beyond the terminals for example in the device under test (DUT) connected to the common terminal. Furthermore, a DC signal applied at the common terminal, for example from a device under test, could alter the switching state of the diplexer. If this DC charge was blocked by a blocking capacitor the diplexer would not be suitable for operating at low frequencies down to DC (direct current). In addition to this, so-called video crosstalk caused by transient switching disturbances is disadvantageously not suppressed. In an example of this document, the switching elements are isolated from the transmission path via a capacitor, the switching elements are however no semiconductor devices. This means that the switching time is rather long.
Accordingly, there is a need to provide an electronically switchable diplexer with low insertion loss, which, on the one hand, avoids DC charge on the terminals and the switching state of which is immune to a DC signal applied to the common terminal, but still allows for operating at low frequencies down to DC, and on the other hand, ensures low video crosstalk by suppressing transient switching disturbances. Additionally, the usable frequency bands of the diplexer should overlap each other, so that there is no gap between the bands where the insertion loss is high.